A telecommunications access network links via access lines the individual subscribers' equipment (e.g. telephone set or other user terminal equipment) with the public switched telephone network. The network comprises mainly of access lines leading from the customer premises to a local telephone exchange. Either at the telephone exchange or at some intermediate concentration point, each access line is terminated at a line interface.
The conventional line interface comprises a telephone system subscriber line interface circuit (SLIC) and a subscriber line audio-processing circuit (SLAC). The SLIC connects a balanced two-wire transmission path (the path to and from the subscriber telephone set) with an unbalanced four wire transmission path (the connection with the telephone exchange). The SLIC performs various functions, including battery feed, supervision function, over voltage protection, ringing, signaling, hybrid (two-to-four wire conversion), timing, and ring-trip detection function. The functionality of a SLIC is often expressed as “BORSHT” (Battery, Over-voltage, Ringing, Supervision, Hybrid and Testing). The battery feed function supplies DC current to the telephone sets to allow for telephone communication. Part of the supervisory functions involves monitoring whether the telephone sets are either on-hook or off-hook.
The exchange (or switch) is usually a digital exchange. In legacy networks this may be a digital local exchange (DLE), handling traffic to and from a plurality of concentrators, each concentrator handling a plurality of individual access lines. In next-generation networks (NGN), the exchange may be a call server handling traffic to and from a plurality of MSANs: each MSAN handling a plurality of individual access lines. In the NGN, the plurality of MSANs is controlled for call handling purposes by a call server and managed for configuration, administration and fault handling purposes by an element manager, whereas, in the legacy case, the plurality of concentrators are controlled by the DLE. The coupling between the SLIC and the exchange normally involves pulse code modulation (PCM), where the signals pass to the exchange through a CODEC and other audio processing (typically provided in the SLAC). SLIC and SLAC design is well known and need not be considered in further detail.
Typically, in telecommunications access networks, lines connecting a telephone set to a line interface may share physical space with other such lines, including passing through the same access cable and cable joints. Each line comprises two wires: known as the A-wire and the B-wire. Manual operations, such as joining cables and wiring distribution frames, are prone to human error which can lead to misconnections, including cross-connected wires—so-called “crossover fault”. A crossover fault occurs when one of the wires from a two-wire access line is inadvertently crossed with a wire from a different two-wire access line. The crossover may involve two A-wires or two B-wires. There is therefore a need for a way to reliably detect occurrences of the crossover fault conditions in the access network.